1. Field of the Invention
The present invention relates to a multilayer ceramic capacitor that uses a dielectric porcelain composition whose primary constituent is CaZrO3 and internal electrodes whose primary constituent is Cu, as well as a method of manufacturing such multilayer ceramic capacitor.
2. Description of the Related Art
Dielectric porcelains whose primary constituent is CaZrO3 have traditionally been used for high-frequency dielectric resonators, filters, multilayer ceramic capacitors, etc. Ideally these multilayer ceramic capacitors, etc., have a smaller temperature coefficient of dielectric constant so that they can be used for modern devices of higher frequencies (approx. 100 MHz to 2 GHz). For the internal electrodes of multilayer ceramic capacitors, which require a low ESR (equivalent series resistance), small loss in the high-frequency range (high Q value), and lower cost, a base metal having a small specific resistance must be selected and accordingly Cu is used instead of Ni or Pd. On the other hand, the dielectric must be a non-reducing material that can be sintered at low temperatures of 1080° C. or below and prevents oxidation of Cu, partly because the dielectric must have a high Q value, small temperature coefficient of dielectric constant, and high reliability, and partly because Cu is used for the internal electrodes. In addition, ideally, use of a dielectric free from Pb and Bi is desired in order to reduce impact on the environment
Dielectric porcelain compositions meeting the aforementioned requirements are already known, and use of such dielectric porcelain compositions for multilayer ceramic capacitors is described in multiple patent literatures.
For example, Patent Literature 1, which relates to a dielectric porcelain composition used for a multilayer ceramic capacitor using Cu as its electrode material, describes that, by using (Ca1-xMgx)(Zr1-y,Tiy)O3 as a primary constituent and adding a glass composition of aSiO2-bLiO1/2-cBO3/2-dCaO-eBaO together with MnO2 by specified quantities, a dielectric porcelain composition that can be sintered at low temperatures to provide a dielectric offering good characteristics including volume resistance can be produced. However, longevity traits of the multilayer ceramic capacitor are not fully examined.
Additionally, Patent Literature 2 describes a dielectric ceramic composition that uses Cu for its internal electrodes and can be sintered at 950° C., wherein such dielectric ceramic composition contains 100 parts by weight of a primary constituent of (Ca1-xRx)(Zr1-yTiy)O3, 0.5 to 2.5 parts by weight of a glass composition of aSiO2-bB2O3-cLi2O-dK2O-eCaO-fAl2O3-gTiO2-hZrO2, and 1.0 to 5.0 parts by weight of a Mn compound, where R in the foregoing is either Mg or Sr as selected and 0≦x≦0.1, 0≦y≦0.1, a+b+c+d+e+f+g+h=100, 20≦a≦35, 20≦b≦35, 20≦c≦30, 3≦d≦5, 2≦e≦12, 2≦f≦10, 1≦g≦12, and 1≦h≦7 are satisfied. However, longevity traits of the resulting multilayer structure are not fully considered.
Furthermore, Patent Literature 3 describes a multilayer ceramic capacitor comprising: a ceramic material containing primary-phase particles whose primary constituent is a CaZrO3 compound as well as secondary-phase particles containing at least Mn; internal electrodes which are embedded in the ceramic material and whose primary constituent is Cu; and external electrodes which are formed on both ends of the ceramic material and each connected electrically to one end of the internal electrodes; wherein Mn is contained by 2 mol or more and Si is contained by 0.69 mol or more in the ceramic material relative to 100 mol of the primary constituent; the area ratio occupied by the secondary-phase particles on a cross section of the ceramic material is 0.1% or more; and at least 67% of the secondary-phase particles whose particle size is 0.1 μm or more contain Cu and Si; and in such multilayer ceramic capacitor, both low-temperature sintering property and longevity traits are considered. However, achieving this requires a more complicated manufacturing process that includes: stacking multiple ceramic green sheets on which a conductive pattern has been formed, to form a ceramic laminate; sintering the ceramic laminate to form a ceramic material; applying a conductive paste for external electrodes on both ends of the ceramic material, followed by baking; and heat-treating the baked ceramic material in a reducing ambience at a temperature of 700° C. or above.
In light of the aforementioned circumstances, the inventors examined ways to improve the longevity traits of a multilayer ceramic capacitor using Cu for its internal electrodes.
For example, Patent Literature 4 proposes a dielectric porcelain composition expressed by CaxZrO3+aMn+bLi+cB+dSi, wherein the contents of the respective constituents are 0.5≦a≦4.0 mol and 6.0≦(b+c+d)≦15.0 mol relative to 100 mol of CaxZrO3 (where 1.00≦x≦1.10) and wherein 0.15≦(b/(c+d))≦0.55 and 0.20≦(d/c)≦3.30 are satisfied, based on the discovery that the Ca/Zr ratio and Mn, Li, B, and Si contents are factors determining the longevity of a CaZrO3 dielectric porcelain and that, when the Ca/Zr ratio and Li—B—Si composition ratio meet certain conditions, the CaZrO3 dielectric porcelain can be made denser at the melting point of Cu of 1080° C. or even below, even when the Li and B contents are reduced in order to keep the longevity from becoming shorter as a result of use of Cu internal electrodes.
In addition, the inventors examined ways to improve the longevity traits further and consequently discovered that, in the case of a system to which Si, B, Li, etc., have been added as secondary constituents of the aforementioned dielectric composition, Ca (alkali earth metal) elutes, in the sintering step, from the primary constituent of CaZrO3 compound to the liquid phase which is considered to have been formed by Si, B, and Li that have been added as secondary constituents, and consequently a secondary phase in which excessive Zr is present is produced and this contributes to the deterioration of longevity traits. In particular, the inventors revealed that the presence of the secondary phase in which excessive Zr is present would have greater impact on the deterioration of longevity when a small amount of Mn, or 5.0 mol or less to be specific, has been added and therefore found that the longevity traits could be improved while keeping the specific dielectric constant and temperature coefficient of volume low by suppressing the abundance of the secondary phase (secondary phase containing more Zr than the primary constituent mentioned above) (Patent Literature 5).